UV map / model turned black on saving - how to recover?
This is a known limitation, currently images are not saved with the rest of the .blend file. This means that reloading the .blend or even undoing in some modes will cause unsaved changes to textures to be lost (Global undo works by reloading an older version of the .blend).
To save your images, press F3 to save it to your hard drive, and AltS to save changes to an existing image on your hard drive. Or press Image editor &gt; Header &gt; Image &gt; Pack to embed it in the .blend, and press Pack again to save changes to an embedded file:

You can tell when your image has unsaved changes because there is a asterisk after the image menu in the header:


As GiantCowFilms mentioned, there is a patch which adds autosave functionality for images. It has not currently been decided whether it should be included or not:

Note, I'd like to have some better way to do this then simply adding another option, it seems a but weak and doesn't really fit into painting workflow.
But doing this by default is a bit risky too.